As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2013 and are well known to the person skilled in the art.
For any description of aluminium alloy compositions or preferred aluminium alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Aluminium alloys comprising lithium are very beneficial for use in the aerospace industry since the purposive addition of lithium may reduce the density of the aluminium alloy by about 3% and increase the modulus of elasticity by about 6% for each weight percent of lithium added. In order for these alloys to be selected in airplanes, their performance with respect to other engineering properties must be as good as that of commonly used alloys, in particular in terms of the compromise between the static mechanical strength properties and the damage tolerance properties. Over time a wide range of aluminium-lithium alloys have been developed with a corresponding wide range of thermo-mechanical processing routes. However, a key processing route remains the casting of ingots or billets for further processing by means of extrusion, forging and/or rolling. The casting process has proven to remain a problematic processing step in the industrial scale production of ingots and billets. There are amongst others issues with regard to oxidation of molten metal in the furnaces, the transfer troughs and during casting itself. And also safety issues remain as “bleed outs” or “runs outs” during casting of aluminium-lithium alloys can lead to much more violent reactions than with non-lithium containing alloys as lithium makes the molten aluminium much more reactive.
U.S. Pat. No. 5,415,220 issued to Reynolds Metals Company discloses a method of direct chill casting of aluminium-lithium alloys under a salt cover to protect the molten metal from oxidation by ambient oxygen, which comprises (a) forming a protective molten salt cover comprising a lithium chloride salt composition in a furnace containing molten aluminium alloy, (b) adding at least one of lithium and a lithium-containing aluminium alloy to the molten aluminium alloy through the salt cover to form a molten aluminium lithium alloy in the furnace, (c) transferring said molten aluminium-lithium alloy to a casting station, and (d) direct chill casting said molten aluminium-lithium alloy into an ingot form such as a billet or a rolling ingot. The molten metal transfer trough may include a metal filter, e.g. a foam filter or a ceramic bed filter designed for both particulate removal and degassing of the molten metal passing through the transfer trough. The molten salt cover is said to be particularly useful in direct chill casting processes wherein a salt cover is added to the ingot head in the mould. The salt mixture includes LiCl, and preferred salt mixtures include LiCl in combination with other salts selected from KCl, NaCl, and LiF. Sodium chloride is less preferred in the melting vessel since the sodium component thereof has a tendency to exchange with the lithium in the aluminium alloy, thereby adversely affecting the alloy content with sodium as a highly undesirable impurity element therein.
The use of salts, or salt mixtures, in the casting of lithium containing aluminium alloys has several disadvantages. An important disadvantage is that the salts are very corrosive for the often applied ceramic foam filters (“CFF”) for removing of any particulate in the molten metal.